1. Field of the Invention
The present invention relates to a diffusing matrix liquid crystal display screen, which screen according to the present invention can be used for terminals, computers, pocket calculators, etc.
2. Discussion of the Background
A display screen essentially comprises two transparent plates retaining between them an electrooptical material, e.g. a liquid crystal film. The attached FIGS. 1 and 2 show an embodiment having such plates.
In FIG. 1 is shown a transparent substrate 10, e.g. of glass, covered:
by a first series of addressing conductive strips 12 arranged in column form, e.g. of indium and tin oxide (ITO), PA1 by an array of electrodes 14, each constituting a display point or pixel, PA1 by a second series of addressing strips 16 arranged in row form and generally constituted by a stack of layers, namely a semiconductor layer (e.g. of aSi:H), an insulating layer (e.g. of SiN) and a conductive layer (e.g. of aluminum).
The electrodes 14 are extended by a finger 20 in such a way that the finger 20 and the column form a source and a drain of a thin film transistor, the gate being constituted by the metal coating of the addressing row.
FIG. 2 shows a second plate with a second transparent substrate 30 (e.g. of glass), blocks of light filters 32 (red, green, blue), a black matrix 34 and a conductive, transparent counter-electrode 36. The light filters 32 and black matrix 34 are not essential to the invention to be described hereinafter.
Such a screen, or at least the first plate illustrated in FIG. 1, can be obtained by a process requiring only two photolithography levels, such as described in FR-A-2 533 072.
Moreover, the known liquid crystal display screens (no matter whether they are or not of the aforementioned type), may operate in a reflective mode. This means that the liquid crystals backscatter the ambient light in the absence of electrical excitation, but become transparent under excitation, allowing a black character to appear.
This operating mode leads to a display in black on a white background. This display is advantageous because it eliminates the rear illumination of the screen, which leads to a corresponding reduction in the electric power consumption of the screen.
The optical effect currently used for obtaining the reflection of the ambient light is linked with the properties of certain liquid crystals of the twisted nematic type used with a rear reflector. However, polymer dispersed liquid crystals (PDLC) are generally used for diffusing ambient light.
The use of twisted nematics requires the use of two polarizers, which absorb more than 505 of the incident light. The brightness (defined as the ratio of the backscattered intensity to the incident intensity) is consequently low. Brightnesses of approximately 5% are normally obtained, which is much too low for high definition displays for video applications. In addition, the contrast is low.
With polymer dispersed liquid crystals, the diffusion is essentially influenced by the optical anisotropy of the liquid crystal, the index of the polymer, the diameter of the liquid crystal droplets and the thickness of the cell. The backscattering of a liquid crystal cell dispersed in a polymer increases when the optical anisotropy of the liquid crystals increases, increases as the index of the polymer approaches the ordinary index of the liquid crystal and increases when the diameter of the drops decreases and when the thickness of the cell increases.
For a given material, e.g. the pair TL205 and PN393 manufactured by MERCK, the indices of the liquid crystal TL205 and the monomer PN393 are given once and for all. Moreover, the diameter of the droplets obtained after irradiation is also fixed to a value between 0.5 and 1 .mu.m. Thus, there is only the thickness of the cell to act on the diffusion of the film of liquid crystals.
With a thickness of 8 .mu.m, the brightness is 12%, whereas at 14 .mu.m the brightness becomes 17%. However, this is obtained to the detriment of the control voltage. At 8 .mu.m, the control voltage is 10 V and rises to close to 20 V for a 14 .mu.m cell. However, the maximum voltage available at the terminals of a screen addressing circuit is approximately 12 V. It is consequently not possible to increase the thickness of the cell in order to increase the brightness of the screen, because this would require voltages beyond the available voltages.